Sea level rise and Antarctica

What’s Natural

Guest post by Jim Steele

California’s and other American coastal towns are engaged in divisive arguments regards rising sea levels. Although observed sea levels rose less than 8 inches (0.08 inches per year) since 1900, some modelers forecast much bleaker futures. They predict a 2.4-foot rise for every 1°F rise above preindustrial temperatures, then accelerating to nearly 4.5 feet for every 1°F additional increase. Why a dramatic acceleration in sea level? It’s based primarily on dire models, typically presented to coastal planning commissions as ‘best science’, suggesting increasing ice instability and Antarctica ice sheet collapse. “Antarctica has the potential to contribute more than 3.3 feet of sea-level rise by 2100 and more than 49 feet by 2500.”

Those models have prompted some citizens to argue we must abandon the coasts via managed retreat. Others argue we should build better sea walls. But how high? Others rightfully ask, “how trustworthy are those models?” Model predictions of a collapsing Antarctica ice sheet are not based on observations.  Models of Antarctica’s catastrophic ice collapse are attempts to explain ancient sea levels such as the 30-foot higher levels 120,000 years ago.

There are good reasons toquestion catastrophic models. For one, away from the coast Antarctica’s surface temperatures average −70 °F. Antarctica’s extremely cold surfaces require global warming to increase many, many times more before surface glaciers could ever melt. For another, although greenhouse theory predicts increasing CO₂  concentrations will raise temperatures, greenhouse theory also predicts added CO₂  has a cooling effect on Antarctica (Wijngaarden & Happer 2020, Schmithüsen 2015).

Up to a point, increasing greenhouse gases  act like a blanket that warms your body by slowing your loss of body heat. Although CO₂  absorbs then rapidly releases heat in less than one-thousandth of a second, at colder altitudes it releases that heat more slowly. Because warmer bodies release more heat than colder ones, the higher and colder atmosphere absorbs the heat released from warmer surfaces faster than it can release heat to space generating the greenhouse effect. In contrast Antarctica’s surface is much colder and the air miles above is warmer.  Warmer greenhouse gases in the air above release heat back to space faster than can be absorbed from the colder surface. Thus, the atmosphere over Antarctica cools faster than if there were no greenhouse gases.

Still, researchers do observe regions of retreating ice. The Antarctic Peninsula was once designated one of the earth’s most rapidly warming regions in the 1980s and 90s, but researchers debated whether melting was caused by global warming or shifting winds. Indeed, warmer winds had been frequently blowing from the north. But the British Antarctic Survey now reports the peninsula has rapidly cooled since the 1990s due to frequent southerly winds from the mainland that can be 50°F to 70°F  colder.  Researchers attribute shifting winds to “extreme natural internal variability”.

Strong winds also cause turbulence that sporadically pulls warmer air from above down to the cold surface resulting in occasional “warming” spikes. Furthermore, strong winds moving over mountains can heat the air simply due to increasing pressure as the winds descend (known as foehn storms).  Without adding heat, the increasing air pressure alone can raise regional temperatures in excess of 72°F eventually causing dry, ice-free regions or causing melt ponds that promote ice shelf collapse.

Finally, because air temperatures rarely reach the melting point (other than during foehn storms), there is no significant melt of Antarctica’s surface ice. However, some glaciers that extend past the coast terminate below sea level and are indeed losing ice. Antarctic oceans consist of a relatively fresh cold layer of surface water that overlays a thick layer of relatively warm salty water known as the Circumpolar Deep Water (CDW).

Antarctic winds can push cold surface water towards the coast and then deeper.  That also pushes the warmer CDW downward and minimizes glacier melting. At other times the winds can  shift and cause surface water to move away from the coast and simultaneously draw up warmer CDW toward the surface. The warm CDW then accelerates the melting of submerged glaciers. Natural oscillations such as El Nino or the Antarctic Oscillation (aka SAM) can shift the winds and induce decades of rapidly retreating glaciers alternating with decades of stable or growing glaciers.

Antarctica’s research community is split 50-50 on whether observed changes are mostly natural or driven by human additions of CO₂. But to date there’s no evidence of an ice sheet collapse that would accelerate sea level rise, and many researchers are walking back the extreme claims of Antarctica’s sea level contributions. Coastal planning commissions would be wise to plan on the same average sea level rise witnessed for the past 100 years but be mindful of Antarctica’s shifting winds and shifting scientific claims.

Jim Steele is Director emeritus of San Francisco State’s Sierra Nevada Field Campus and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism